In general, exhaust gas discharged from an internal combustion engine includes particulate matter (PM) having carbon as the main component and ash including an incombustible component and is known to cause air pollution. In this regard, regulations relating to the emissions of particulate matter and also hazardous components such as hydrocarbons (HC), carbon monoxide (CO), and nitrogen oxide (NOx) contained in exhaust gas have been steadily enhanced year by year. Accordingly, techniques have been suggested for collecting and removing the particulate matter from the exhaust gas.
For example, a particulate filter is provided inside the exhaust passage of an internal combustion engine for collecting the particulate matter. For example, in diesel engines, a certain amount of particulate matter is discharged together with the exhaust gas, and to cope with this, a diesel particulate filter (DPF) is mounted inside the exhaust passage. Such a particulate filter is known to have the so-called wall-flow type structure in which a porous base material is configured to have a large number of cells and inlets and outlets of the large number of cells are alternately closed (Patent Literature 1 and 2). In the wall-flow type particulate filter, the exhaust gas that flows in from the cell inlet passes through the partitioned porous cell walls and is discharged to the cell outlet. While the exhaust gas are passing through the porous cell walls, the particulate matter is trapped in the fine pores inside the walls.
In recent years, the idea of imparting NOx purification capacity to the filter has been investigated to further improve the purification performance. For example, it has been suggested to provide the filter with a SCR (Selective Catalytic Reduction) catalyst for selectively reducing the NOx in the exhaust gas by the reduction action of ammonia or the like (Patent Literature 3). For example, in an exhaust gas purification apparatus of a urea addition type, urea water is supplied upstream of the filter supporting the SCR catalyst, and ammonia is generated by the hydrolysis of the urea water. This ammonia is adsorbed on the SCR catalyst, and the NOx in the exhaust gas is purified by the reduction action of the adsorbed ammonia. Zeolite such as copper-supporting zeolites and iron-supporting zeolites have been used as the SCR catalysts.